The toxic side effects of many therapies, including standard treatments for cancers, effectively limit the amount of agent that may be administered to a patient. Additionally, many agents cause organ-specific toxicities, further limiting the dose that may be delivered to the target tissue. For instance, the cardiotoxicity of many anthracycline family members reduces the maximum therapeutic dose available for this group of chemotherapeutic agents. Targeted drug delivery of various detectable or therapeutic agents can lower toxicity in normal tissue and increase the efficacy of treatment by allowing concentrated localized effects on specific tissues.
Somatostatin, bombesin, or other biologically active peptide analogs have been used to detect tumor cells over expressing receptors specific for the peptides (see for example Denzler and Reubi, Cancer 85(1):188-198, 1999). Somatostatin, bombesin, and many other biologically active peptide agonist analogs are rapidly internalized after binding to their receptors (Lukinius et al, Acta Onc. 38:383-387, 1999; Morel, Biochem. Pharmacol. 47(1):63-76, 1994). This internalization of the peptide analogs may result in translocation to the cell nucleus (Chen et al., Am. J. Physiol. Renal Physiol. 279:F440-F448, 2000; Hornick et al., J. Nucl. Med. 41(7):1256-1263, 2000; Janson et al., J. Nucl. Med. 41(9):1514-1518, 2000).
Somatostatin analogs bind particular somatostatin receptor subtypes that are present on the surface of specific normal or diseased tissues. Somatostatin receptors are up-regulated in specific diseased tissues including inflammatory bowel diseases, rheumatoid arthritis, a variety of tumor types, and blood vessels supplying many tumors, often in a subtype-specific manner. (Denzler and Reubi, Cancer, 85:188-198, 1999; Plonowski et al., Cancer Res. 60(11):2996-3001, 2000; Kahan et al., Int. J. Cancer 82(4):592-598, 1999; Gulec et al., Surg. Res. 97(2):131-137, 2001). Similarly, receptors specific for another biologically active peptide, substance P, can be up-regulated in various diseases (Id.). Somatostatin-related urotensin II peptide receptors have been found to be expressed on a number of neural tumors (Takahashi et al., Peptides 22:1175-1179, 2001). Receptors for GnRH II ligands/analogs have been located on many peripheral tissues of interest including breast, prostate, and the GI tract (Neill et al., Biochem. Biophys. Res. Commun. 282:1012-1019; Millar et al., Proc. Natl. Acad. Sci. USA 98:963609641, 2001).
At least five somatostatin receptors subtypes have been characterized, and tumors can express various receptor subtypes (Shaer et al., Int. J. Cancer 70:530-537, 1997). Naturally occurring somatostatin and its analogs exhibit differential binding to these receptor subtypes, allowing precise targeting of a peptide analog to specific diseased tissues.
The physical and chemical properties of many compounds such as cytotoxic agents make their conjugation to biologically active peptides problematic. The agent or drug may reduce the specificity of binding or the biological activity of the peptide analog, limiting its effectiveness as a targeting agent. Additionally, therapeutic and cytotoxic agents may have chemical properties that cause reduced solubility and promote accumulation of drug-peptide analogs in certain organs, thus increasing toxicity and reducing efficacy. Effective means are needed to link cytotoxic agents to a targeting agent such as somatostatin, bombesin, or another biologically active peptide and to decrease non-target uptake of the cytotoxic agents, while retaining the activity of each component, thus maximizing therapeutic effects and minimizing toxicity.
Furthermore, while iodinization and astatination (see Vaidyanathan et al., Nucl. Med. Biol. 27(4):329-337, 2000) hold great promise for use in imaging and possibly in aiding treatment of diseases associated with increased expression of a factor specific for a biologically active peptide, problems exist with the methods now available for labeling a range of peptide analogs. The use of labeled biologically active peptides has been studied in various systems. Radioactive halogens such as iodine have great potential as tumor imaging and cytotoxic agents. Promising isotopes include 125I (K. S. Sastry, Am. Assoc. Phys. Med. 19:1361-1370, 1992; Mariano, J. Nucl. Med. 41(9):1519-1521, 2000), 131I (Wheldon et al., Radiother. Oncol., 21:91-99, 1991), 123I (Blower et al., Eur. J. Nucl. Med. 25:101-108, 1998; Janson et al, J. Nuc. Med. 41(9):1514-1518, 2000; Mariani et al, J. Nuc. Med. 41(9):1519-1521, 2000), and 124I (Glaser et al., J. Labelled Compd. Radiopharm. 44(6):465-480, 2001). There are several problems associated with the addition of radioactive iodine atoms to peptides (Bakker et al., Eur. J. Nucl. Med. 23(7):775-781, 1996). One is the rapid loss of iodines from L-Tyr residues by specific de-iodination enzymes (Kawai et al., Nucl. Med. Biol. 17(4):369-76, 1990). Another problem is the great increase in hydrophobicity produced by addition of iodine to a peptide agent, which is associated with increased accumulation of radioactivity in the liver, interfering with tumor imaging and promoting severe toxicity. A further problem is loss of binding affinity when tyrosines next to the pharmacophore are iodinated. A linker capable of facilitating labeling of a variety of biologically active peptides without deleterious in vivo accumulation is needed.